Although not restricted to this type of application, the invention will be more specifically described with reference to tires for vehicles of the loader type working in mines and with an axial width in excess of 37 inches. This type of loader is used in mines to fill the skips of vehicles of the dump truck type.
The reinforcing structure or reinforcement of tires, particularly of construction vehicle tires is currently—and usually—made up of a stack of one or more plies conventionally known as “carcass plies”, “crown plies”, etc. This way of naming the reinforcement stems from the method of manufacture which involves producing a series of semi-finished products in the form of plies, provided with thread-like reinforcements, often longitudinal, which are then assembled or stacked in order to build a green tire. The plies are produced flat, with substantial dimensions, and are then cut to the dimensions of a given product. The plies are also initially assembled substantially flat. The green tire thus produced is then shaped to adopt the toroidal profile typical of tires. The semi-finished “finishing” products are then applied to the green tire to obtain a product ready to be cured.
A “conventional” type of method such as this involves, particularly during the phase of manufacturing the green tire, the use of an anchoring element (generally a bead wire), which is used to anchor or retain the carcass reinforcement in the bead region of the tire. Thus, for this type of method, a portion of all the plies that make up the carcass reinforcement (or just some of them) are folded back around a bead wire positioned in the bead of the tire. That then anchors the carcass reinforcement in the bead.
The widespread use across industry of this conventional type of method, in spite of there being numerous variations in how the plies are created and assembled, has led those skilled in the art to adopt a vocabulary based on the method; hence the terminology generally used, involving in particular the terms “plies”, “carcass”, “bead wire”, “shaping” to denote the transition from a flat profile to a toroidal profile, etc.
Nowadays there are tires which do not strictly speaking have “plies” or “bead wires” as understood from the above definitions. For example, document EP 0 582 196 describes tires manufactured without the aid of semi-finished products in the form of plies. For example, the reinforcing elements of the various reinforcing structures are applied directly to the adjacent layers of rubber compounds, everything being applied in successive layers to a toroidal core, the shape of which directly yields a profile similar to the final profile of the tire that is in the process of being manufactured. Thus, in that case, there are no longer any “semi-finished” products or “plies” or “bead wires”. The basic products such as the rubber compounds and the reinforcing elements in the form of threads or filaments are applied directly to the core. As this core is of a toroidal shape, there is no longer any need to form the green tire in order to change from a flat profile to a profile in the form of a torus.
There are also methods of assembly on the toroidal core that employ semi-finished products specially designed for rapid, effective and simple lying onto a central core. Finally, it is also possible to use a hybrid comprising both certain semi-finished products for achieving certain architectural aspects (such as plies, bead wires, etc.), while others are created by applying reinforcing elements and/or compounds directly.
In this document, in order to take account of recent technological advances both in the field of manufacture and in the design of the products, the conventional terms such as “plies”, “bead wires”, etc. are advantageously replaced by terms which are neutral or independent of the type of method used. Hence, the term “carcass type reinforcement” or “sidewall reinforcement” can validly be used to denote the reinforcing elements of a carcass ply in the conventional method, and the corresponding reinforcing elements, generally applied to the sidewalls, of a tire produced according to a method that does not involve semi-finished products. The term “anchoring zone”, for its part, can just as easily denote the “traditional” turning back of the carcass ply around a bead wire in a conventional method as it can the assembly formed by the circumferential reinforcing elements, the rubber compound and the adjacent sidewall reinforcing portions of a bottom region created using a method which involves applying elements to a toroidal core.
As far as the usual design of tires for construction plants is concerned, the radial carcass reinforcement anchored in each bead is made up of at least one layer of metal reinforcing elements, the said elements being substantially mutually parallel within the layer. The carcass reinforcement is usually surmounted by a crown reinforcement made up of at least two working crown layers of metal reinforcing elements, but which are crossed from one layer to the next, making angles of between 15 and 70° with the circumferential direction. Between the carcass reinforcement and the working crown layers there are usually two layers of reinforcing elements, crossed from one layer to the next, and at angles of less than 12°; the width of these layers of reinforcing elements is usually less than that of the working layers. Radially on the outside of the working layers there are then protective layers, the reinforcing elements of which are at angles of between 10 and 65°. The crown reinforcement is itself surmounted by a tread.
The term “axial” means a direction parallel to the axis of rotation of the tire and the term “radial” means a direction intersecting the axis of rotation of the tire which is perpendicular thereto. The axis of rotation of the tire is the axis about which it rotates in normal use.
A circumferential plane or circumferential plane of section is a plane perpendicular to the axis of rotation of the tire. The equatorial plane or circumferential mid-plane is the circumferential plane that passes through the centre or crown of the tread and which divides the tire into two halves.
A radial plane is a plane which contains the axis of rotation of the tire.
The longitudinal direction of the tire, or circumferential direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
Construction plant tires like those described hereinabove are usually subjected to a pressure of between 4 and 10 bars for normal sizes and loads.
In the case of tires for loaders as described hereinabove, the crown reinforcement of the tire is special in so far as the said loaders are articulated vehicles which may be fitted with tires with very low cornering rigidity. The crown reinforcements thus defined for this type of tire, which in particular have working layers, the reinforcing elements of which make angles in excess of 45° with the circumferential direction, are particularly advantageous in terms of tire wear for the uses associated with this type of vehicle.
By contrast, under particularly harsh conditions of use of these tires, the latter may suffer damage to the working layers of the crown reinforcement in the zones at the ends of the layers of reinforcing elements at angles smaller than 12°.